1. Technical field of the Invention
This invention relates to an insert molding die, an insert molding apparatus and an insert molding method.
2. Description of the Related Art
A sensor, a valve part, etc., are conventionally produced by covering a functional part requiring waterproofness such as an IC or an electric coil with a resin housing by injection molding and giving the waterproofness to the resin housing by use of the welding technique or the like.
On the other hand, a method is available in which an IC or an electric coil is inserted and covered with a pipe during the pipe forming process by extrusion molding, for example, and clamped by suction of the internal air before cooling while sealing the opening of the pipe by welding. This method greatly reduces the cost and can seal by molding, for example, relatively fragile parts (glass pipes, bare chips, etc.) which cannot be inserted in the normal insert molding.
The insert molding which belongs to the injection molding high in productivity is generally used to produce a functional part (such as a sensor or a valve part). For insertion of an electric coil or an IC, some waterproof structure is required for a molded part making up the part housing. For this purpose, the heatpin-draw molding technique disclosed in Japanese Unexamined Patent Publication No. 9-38982 or the mold/welding method disclosed in Japanese Unexamined Patent Publications No. 14-200643, for example, is used to impart waterproofness to the internal insert part.
In the injection molding, in order to inject the molten resin into the die in the injection process and hold the shape accuracy by compensating for the volume reduction due to the phase change in the resin solidification process, a high pressure is imparted to the resin in the die in the dwelling process. This poses the problem that a high-strength die is required and fragile parts cannot be inserted.
To cope with this problem, U.S. Pat. No. 3,979,301 discloses an insert molding method in which the insert is held in an extruded pipe and shaped by air suction or clamp to eliminate the need of the high pressure required in the conventional injection molding, thereby making it possible to shape the fragile insert and reduce the cost due to the reduced rigidity of the die.
This insert molding method is explained. The processes for carrying out this insert molding method include (1) the positioning process, (2) the arrangement process and (3) the shaping process.
First, as shown in FIG. 18, a die 1 is prepared, and with the die 1 open, the positioning process is executed by setting an insert part 2 is set at a predetermined position in the die 1 (FIG. 19).
In this case, the die 1 includes a pair of die pieces, each having a forming surface 1a corresponding to the outer shape of the insert part 2. A die 3 is added to the die 1. The die 3 has a shaping portion 4 of a predetermined shape constituting a path of a molten resin material m having a pipe-shaped cross section to extrude the molten resin material m in the die 1.
Further, a heater 5 constituting a welding means is arranged at a predetermined position of the die 1.
The insert molded part 2 is supported on a support unit 6 at a predetermined portion not covered or shaped by the molten resin material m, and set in position as predetermined in the die 1 by a robot or the like having the support portion 6 (FIG. 19).
Next, after positioning the insert part 2, as shown in FIG. 20, the arrangement process is executed in which the molten resin material m is extruded into the die 1 from the die 3 with the die 1 open while at the same time arranging the insert part 2 in the internal space of the molten resin material m.
The molten resin material m extruded from the die 3 has a pipe-shaped cross section due to the shape of the die 3, and has an open forward end portion extruded from the die 3. In the process, the insert part 2 is fixed at a position in the internal space of the molten resin material m having the pipe-shaped cross section, and therefore, the insert part 2 is inserted into the internal space from the open side of the molten resin material m by the extrusion of the molten resin material m.
Then, the shaping process is executed in which the die 1 is clamped on the molten resin material m with the insert part 2 arranged in the space thereof, so that the molten resin material m shown in FIG. 21 is covered and shaped on the insert part 2.
The die 1 has a forming surface 1a corresponding to the outer shape of the insert part 2 as a surface in contact with the molten resin material m, and therefore, the molten resin material m is deformed into the shape corresponding to the forming surface 1a of the die 1 by clamping while the air in the internal space is discharged from the open side (FIG. 22).
According to the insert molding method described above, the molten resin material m is shaped by the die 1 having the forming surface 1a corresponding to the outer shape of the insert part 2, and therefore, the dimensional accuracy of the insert mold 7 formed is considered to be improved (FIG. 23).
In the conventional method described above, however, as shown in FIG. 24, the gap between the molten resin material m extruded in the shape of pipe and the insert holding die 1 cannot be reduced to zero, so that the air leaks from the gap G (FIG. 25) at the time of air suction (FIG. 26), thereby posing the problem of a reduced shaping efficiency or a reduced air suction efficiency, resulting in a lower productivity. The air leakage reduces the pipe temperature, thereby posing another problem of a reduced welding efficiency due to the heat press at the time of clamping. Still another problem is that in spite of the fact that the pipe shaping stability is improved generally by the continuous extrusion in the extrusion molding, the conventional structure requires the intermittent extrusion, resulting in a lower shape stability and a lower productivity.